1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a method for manufacturing the same, and more particularly, to a color liquid crystal display in which the thickness of orientation films in the LCD is adjusted to compensate for the color properties of the LCD.
2. Discussion of the Related Art
Typically, a thin film transistor liquid crystal display (TFT-LCD) includes an upper substrate and a lower substrate placed opposite to the upper substrate. The two substrates maintain a designated cell gap between them, and a liquid crystal is injected between the upper substrate and the lower substrate and sealed there-between.
FIG. 1 illustrates a structure of a TFT-LCD.
The lower substrate is formed with a polarizing plate 1, a transparent substrate 2, a TFT array 3 and an orientation film 4.
On the transparent substrate 2, a plurality of TFTs and a plurality of pixels are arranged in a matrix formation, with each pixel having a pixel electrode connected to a TFT as a basic unit. Also, a plurality of gate buslines and data buslines that are electrically connected to one another are formed at each TFT.
A gate electrode of the TFT formed around the intersecting point of the gate busline and the data busline diverges from the gate busline, and a source electrode diverges from the data busline.
The upper substrate includes a polarizing plate 5, a transparent substrate 6, a color filter 7, a common electrode 8, and an orientation film 9.
The color filter 7 has at least one of red, green and blue dyes and comprises a plurality of pixels. The common electrode 8 and the orientation film 9 are formed sequentially on the transparent substrate 6.
A sealed liquid crystal 10 is injected between the upper and lower substrates, and a spacer 11 is placed between the two substrates to maintain a constant cell gap.
In order to exhibit an intended image, the liquid crystal display having the above construction permits a voltage to be applied to the pixel and generates a voltage difference between the pixel electrode located in the pixel and the common electrode of the color filter substrate, rearranging a corresponding part of the liquid crystal.
In other words, if a voltage is applied to a gate busline and a data busline, respectively, then only the TFT where the gate voltage is applied is turned on. And the voltage of the data busline causes electric charges to be accumulated in the pixel electrode connected to a drain electrode of the TFT which has been turned on. Consequently, the voltage is applied to the liquid crystal between the pixel electrode and the common electrode only. As a result, the arrangement of liquid crystal molecules is changed and light is transmitted or blocked according to the specific arrangement. In this way, the intended image is displayed by selectively controlling the transmission or blocking the light to each pixel.
The liquid crystal display uses color filters corresponding to three primary colors of light, i.e., Red (R), Green (G) and Blue (B) in order to display a color. The RGB color filters are arrayed close to one another, and an appropriate color signal is applied to a corresponding color filter to control the luminance of light.
Once again, the procedure for displaying a color involves a number of steps. That is, the luminance of each light is controlled by changing the arrangement of the liquid crystal molecules, and the voltage necessary to drive the liquid crystal is outputted from a source driver integrated circuit (IC) and supplied through the pixel TFT.
The data voltage supplied to each pixel in this way changes the arrangement of the liquid crystal layer, which consequently affects light transmittance due to the rearranged liquid crystal layer between the upper and lower polarizing plates. Here, the number of colors that is possible to be displayed depends on the number of steps taken to control the liquid crystal. For example, in the case of NW (Normally) ECB mode, when no voltage is applied, it becomes a white state, whereas when the highest voltage is applied, it becomes a black state. Assuming that there is a color filter per pixel, if a medium voltage is applied to the color filter of the pixel, it is possible to display a medium tone according to the voltage applied, and to control the brightness and the chroma of light.
The color screen of the TFT-LCD displays a white light emitted by a back light (B/L), and a mixture of three primary colors transmitted from the R, G and B color filters through each pixel. A TFT is used to drive the liquid crystal cell of each pixel, thereby determining the amount of light transmitted by the liquid crystal cell.
The color filter is made from organic materials such as dyes or pigments. The manufacturing methods include a dye method, a dispersion method, an electro-deposition method, and a print method. The most common method among them is the pigment dispersion method, which is widely used for manufacturing the color filter of the TFT-LCD.
Normally, pigment particles are opaque because they disperse light. However, if the size of the particles is smaller than the wavelength of the light, the particles transmit light and become transparent. Thus, it is better to have smaller particles in order to obtain a high degree of transparency and an excellent dispersion property.
As explained before, the liquid crystal display is an electro-optic element driven by the voltage applied to liquid crystal molecules that are injected to the constant cell gap between the upper substrate and the lower substrate.
Therefore, it is very important to maintain a constant gap between the two substrates.
If the gap between the two substrates is not constant, the transmittance of light passing through the part is also changed, so a uniformity in the brightness in different portions of the device is not obtained.
On the other hand, even if the cell gap is maintained constant, there still could be a color balance problem.
Hereinafter, the aforementioned problem is described in more detail.
AFLC mode, ECB mode and IPS mode are sometimes called a birefringence mode because all of them take advantage of the birefringence property of light in principle. The light transmittance (T) in such a mode can be generally calculated by the following equation: T=Sin2(2θ)*Sin2(δ/2) (wherein, θ is an angle between a transmission axis of a polarizing plate to the side of an incident light and the liquid crystal direction; δ (phase contrast)=2πd*Δneff/λ; d is a cell gap; Δneff is an effective refractive index; and λ is a wavelength of light).
From the equation, it is apparent that the transmittance (T) is related to the cell gap (d), the wavelength of light (λ) and a structure of the liquid crystal layer. Looking at the transmittance properties at each color pixel according to the applied voltage and assuming that the cell gap (d) stays constant, it is found from the light transmittance T=Sin2(2θ)*Sin2(δ/2) that the phase difference is different for each color even with the same arrangement of the liquid crystal layer. Consequently, the transmittance in gray levels are different for each color. Therefore, the color balance in gray levels is destroyed. In addition, the voltage required for generating the maximum transmittance is differently applied to each R, G and B pixel, making it more difficult to achieve the optimum white light in general. One of the attempts made to solve this problem is to apply different cell gaps to the R, G and B pixels, respectively.
As shown in the equation described above, i.e., δ=2πd*Δneff/λ, in order to make the phase difference (δ) equal, the cell gap of each color pixel should be optimized respectively, that is, as the wavelength gets longer, the value of d*Δneff should be larger also. However, since Δneff depends on the medium properties, the cell gap (d) is a variable that can be changed. Thus, the cell gap (d) should be larger to maintain the phase difference at the same value even when the wavelength becomes longer. In other words, if the cell gap (d) is different for each color, the transmittance difference in gray levels during applying the voltage and the color balance problem thereof can be solved together.
FIG. 2 shows the main parts of the liquid crystal display in the related art where the cell gap (d) is different for each R, G or B color. Here, the liquid crystal display includes two transparent substrates 201 and 202 that oppose each other, two orientation films 208 and 209 between the substrates, a spacer 205 between the orientation films to maintain a designated gap where liquid crystal 206 is injected and sealed, and a color filter 207 (R, G, B) with different thicknesses, respectively, in compensation for color balance.
Namely, when applying the voltage to the liquid crystal, the color balance in the middle gray is sometimes lost because of the slope of transmittance of each color (TR, TG, TB) is different from one another. The problem is overcome by varying the cell gap of each color (dR, dG, dB).
Unfortunately, since the technology of the related art differentiates the thicknesses of each color filter from one another as a way of reinforcing color properties, it is not appropriate to obtain the gap uniformity and the orientation stability because of a big step in the R, G and B pixels, respectively. When the orientation is unstable for the reason stated above, any unnecessary small domain can be generated, and the orientation itself during rubbing is not easily done when compared with the flat substrate. Moreover, the orientation around the borders having the above-mentioned step is relatively unstable compared with other surrounding areas, generating a disclination between the domains and consequently decreasing C/R.